Nuclear Magnetic Resonance imaging is fast becoming a most powerful method for the non-invasive diagnosis of disease. A fundamental limitation of the technique derives from the fact that images are constructed from T1 relaxation time measurements of protons in the various biological "compartments". If T1 values for differing soft tissue types are similar, the type will not, in general, be resolvable in the images. A potential method for improving this situation is the development of relaxation agents which specifically after Tl relaxation rates in tissues where they may be concentrated. We propose to design and construct such in vivo relaxation agents. A study of concentration dependence of Tl relaxation by various metal chelates and organic nitroxyl radicals has been prepared. Based on these studies, the metal chelates appear to be more efficient.